There is a need in various fields for technology for non-contacting determination of the relative positions of two parts or items movable relative to one another. These can be parts of precision machines whose positions are to be established with a high level of accuracy, or instrument dials, e.g., reading optical instruments. Thus, e.g., in the case of conventional levelling instruments or levels using a telescope or television camera connected to the instrument, a height reading scale on a remote levelling rod or staff must be read. However, the reading of the remote height reading scale via the telescope requires a relatively high degree of concentration on the part of the observer. Thus, reading errors constantly occur with this method, particularly where the observers are untrained.
Attempts have been made to eliminate the high susceptibility to error of known levels in different ways. Japanese Specification No. 57749-81 proposed providing the levelling staff with coded light sources corresponding to the height on the reading scale. The reading of the staff is to take place with the aid of a receiver adapted thereto. However, the levelling staff is very complicated and is not really suitable for rough use in the open or on building sites. In addition, this levelling staff has a considerable energy consumption, which is also disadvantageous.
According to Australian provisional patent No. 2542/77, the levelling staff is provided with combinations of black and white longitudinal strips and triangles. By scanning these marks at right angles to the staff and on choosing a suitable code, conclusions can be drawn with regard to the measured height from the scanned pulse length. However, in this case, the limited measuring accuracy and the low sensitivity when making readings over longer distances are severe disadvantages.